JEE Main & Advanced Physics Nuclear Physics And Radioactivity JEE PYQ-Nuclear Physics

  • question_answer
    At some instant, a radioactive sample\[{{S}_{1}}\] having an activity\[5\mu Ci\] has twice the number of nuclei as another sample\[{{S}_{2}}\]which has an activity of \[10\mu Ci.\]The half-lives \[{{S}_{1}}\]of and \[{{S}_{2}}\]are [JEE Main Online 16-4-2018]

    A) 10 years and 20 years, respectively

    B) 5 years and 20 years, respectively

    C) 20 years and 10 years, respectively

    D) 20 years and 5 years, respectively

    Correct Answer: B

    Solution :

    [b] For sample 1 we can write,
                \[{{N}_{1}}={{N}_{o1}}{{e}^{-{{\lambda }_{1}}t}}\]
                \[{{N}_{2}}={{N}_{o2}}{{e}^{-{{\lambda }_{2}}t}}\]
    Given that at a particular time t\[{{N}_{1}}=2{{N}_{2}}\]
    Also given that
    For sample 1,
    \[{{A}_{1}}=-\frac{d{{N}_{1}}}{dt}={{N}_{o1}}{{\lambda }_{1}}{{e}^{-{{\lambda }_{1}}t}}=5\mu {{C}_{i}}\]
    \[{{A}_{2}}=-\frac{d{{N}_{2}}}{dt}={{N}_{o2}}{{\lambda }_{2}}{{e}^{-{{\lambda }_{2}}t}}=10\mu {{C}_{i}}\]
    Solving above equations we get,
    \[\frac{{{\lambda }_{1}}}{{{\lambda }_{2}}}=\frac{1}{4}\]
    \[\frac{half\,life\,{{e}_{1}}}{half\,life\,{{e}_{2}}}=\frac{\ln 2}{{{\lambda }_{1}}}\times \frac{{{\lambda }_{2}}}{\ln 2}=4\]


You need to login to perform this action.
You will be redirected in 3 sec spinner